The epidermis is an epithelium, conventionally divided up into a basal layer of keratinocytes containing, in particular, skin stem cells and constituting the germinative layer of the epidermis, a “spiny” layer constituted of several layers of polyhedral cells placed on the basal layer, a “granular” layer comprising one to three layers said to be of flattened cells containing distinct cytoplasmic inclusions, keratohyalin granules, and finally, a set of upper layers, called horny layer (or stratum corneum) constituted of keratinocytes at the terminal stage of their differentiation, called corneocytes.
The stratum corneum, or horny layer, is the superficial layer of the epidermis located at the interface between the organism and its environment. It is composed of corneocytes, which are anucleate cells resulting from the differentiation of epidermal keratinocytes. The corneocytes are rich in keratins and are surrounded by an impermeable lipid matrix. By virtue of its protein and lipid composition, the stratum corneum plays an essential skin barrier role. It prevents the intrusion of microbiological agents and makes it possible to preserve the hydration of the skin and therefore of the body in general.
During skin aging, and other than the well-known consequences of age on the relief of the skin, many forms of discomfort are reported by elderly individuals. These forms of discomfort are caused by an impairment of the barrier function and of the epidermal homeostasis of their skin. Thus, the stratum corneum of aged skin has a reduced intercellular lipid content compared with young skin, particularly during the winter period. This change in composition of the stratum corneum disrupts its physicochemical skin barrier properties. Finally, the speed of recovery of the barrier function after impairment of the stratum corneum slows down with age, implying a dysfunction of the homeostatic function of the epidermis (Denda, M., 2002; Ghadially, R. et al, 1995; Leveque, J. L., 2001). The Applicant company has, in addition, observed, under these same circumstances, that, in response to a physical or chemical attack on the stratum corneum, particular genes show modulation kinetics that are significantly different according to the age of the individual. More specifically, the induction of the expression of certain genes is found to be slowed in aged skin, in comparison with young skin.
Among these genes of which the expression is particularly slowed in elderly individuals exposed to a stress is in particular the keratin 6B gene (KRT6B) which has, moreover, been characterized as particularly advantageous from the viewpoint of its involvement in epidermal repair and regeneration processes.
Moreover, the homeostasis of the skin, and in particular of the epidermis, results from a finely regulated balance between the processes of proliferation and differentation of the cells of the skin. These proliferation and differentiation processes are perfectly regulated: they participate in the renewal and/or regeneration of the skin and lead to the maintenance of a constant skin thickness, and in particular a constant epidermal thickness. This homeostasis of the skin is also involved in maintaining the mechanical properties of the skin.
However, this homeostasis of the skin can be impaired by certain physiological factors (age, menopause, hormones, etc.) or environmental factors (UV stress, pollution, oxidative stress, irritant stress, etc.). The regenerative potential of the epidermis becomes less great: the cells of the basal layer divide less actively, which leads in particular to a slowing down and/or a decrease in epidermal renewal. Consequently, cell renewal no longer compensates for the loss of the cells removed at the surface, leading to atrophy of the epidermis and/or to a decrease in the thickness of the skin and/or a loss of elasticity and/or of firmness of the skin.
The alterations in epidermal homeostasis are also reflected by a dull and/or poorly defined appearance to the complexion of the skin.
This phenomenon may be accentuated by the menopause: women complain of their skin tightening and becoming dry, or even of the appearance of xerosis. The hormonal deficits associated with the menopause are accompanied in particular by a drop in metabolic activity, which could result in a decrease in the proliferation of the keratinocytes and an increase in epidermal differentiation.
It is therefore advantageous to also have available compositions capable of promoting the homeostasis of the skin in order to maintain and/or increase the thickness of the skin and thus to maintain and/or improve the mechanical properties of the skin and/or promote the radiance of the complexion.